One common use for a metallic conductor communications cable is the routing in underground ducts in metropolitan areas. In order to maintain performance of the communication cable, various means of protection must be provided. For example, the cable must be provided with mechanical protection to withstand abuse during handling and installation as well as during use. Also, the cable must be protected against stray electromagnetic fields.
An inner shield which is formed about an inner jacket disposed about the core and made of a material having a relatively high electrical conductivity is used to dissipate stray currents due to electromagnetic fields or lightning, for example. Formed about the inner shield which may comprise aluminum, for example, may be an outer shield which is made of a material such as steel having a relatively high modulus of elasticity. The outer shield provides suitable mechanical strength for the cable. A plastic outer jacket is commonly provided about the outer shield.
In order to preserve the transmission qualities of a communications cable, it becomes necessary also to prevent the ingress of moisture into a multiconductor core of the cable. This may be accomplished by introducing a pressurized gas into what is referred to as an air-core cable. In another technique, the cable core is filled with a waterproofing material after which a metallic shield having its major surfaces flooded with a waterproofing material is wrapped about the core. The last-described technique produces what is referred to as a filled cable and avoids the necessity of pressurizing the cable.
It is commonplace to use a pressurized cable core in underground cable duct systems. Dry air, typically at 10 psi, is pumped into the cable. Such an arrangement has proven to be effective in preventing the ingress of water.
It is not uncommon to bond the plastic outer jacket to the outer surface of the outer shield. The resulting product is referred to as a bonded sheath cable. By bonding the plastic jacket to the outer shield, which generally is corrugated, it has been found that the resistance of the cable to moisture diffusion is increased substantially. Further, if the jacket is not bonded to an adjacent shield, the pulling of the cable into an underground duct may cause a separation of the jacket from the shield. The bonding of the jacket to the outer metallic shield provides a composite which has enhanced strength when subjected to bending and/or to torsion.
In the manufacture of a bonded sheath cable, a metallic tape such as a steel tape is precoated on one major surface with a layer of an adhesive polymer material. Then the tape may be wrapped about an inner shield, which for a filled cable has been flooded with a waterproofing material, and the core to form an outer shield having a longitudinal overlapped seam. An overlying edge portion of the outer shield is directed inwardly toward the core. When a plastic jacket is extruded over the shielded core, heat from the semi-molten plastic material causes the adhesive layer on the outer surface of the shield to bond the jacket to the outer shield. This provides a laminate which is a composite of steel and plastic that reduces substantially the ingress of moisture into the core. Also, it provides mechanical strength to resist buckling and crushing.
Commercially available bonded sheath cables have been found to have a problem associated therewith. Splitting of the plastic jacket has occurred in bonded sheath cables. Typically, this has occurred along the overlapped seam of the outer shield particularly along an outer longitudinally extending free edge of the outer overlapping portion of the outer shield. When the jacket splits, the mechanical integrity of the cable is compromised. Further, paths are formed by by which water may enter the cable and then run longitudinally, perhaps into closures.
In the prior art, attempts have been made to correct this problem. For example, one design includes a tape which is disposed across the seam contiguous to the outer surface of the outer shield. This arrangement has not seemed to abate the splitting of the jacket. In another arrangement, a bead of a plastic material is caused to be applied along the overlapped seam of the outer shield. The bead of plastic material bonds to the overlying jacket and to the underlying shield. It appears that the bead of plastic material retains its configuration as applied even after the jacket is extruded thereover. As far as is known, this proposed solution has not been totally effective in preventing jacket splitting.
What is needed and what seemingly is not provided by the prior art is a cable having a bonded sheath system in which the integrity of a jacket bonded to a shield is maintained notwithstanding bending and/or twisting of the cable. In particular what is needed is a bonded sheath system in which jacket splitting along a longitudinal seam of the shield during bending and/or twisting is avoided.